This invention relates to a developing device for an electrophotographic apparatus such as a copier and a laser printer.
A developing device for a color copier which forms a background of the present invention is shown in FIG. 4 wherein numeral 1 indicates a developer tank inside which developing agent is circulated along the dotted line with arrows around two stirrer rollers 2 and 3, a developer roller 4 which is a magnet roller, and a developing agent transporting roller 5 which is another magnet roller while toner in the developing agent is supplied to a photoreceptor (not shown) by the developer roller 4 through an opening 1a provided at the bottom of the developer tank. Surrounding the stirrer roller 3 is a cylindrical screening member 6 with openings 6a and 6b along portions of its circumference. This screening member 6 which is disposed rotatably with respect to the stirrer roller 3 such that the developing agent can be circulated freely during a development period as shown in FIG. 4 but that its flow from the stirrer roller 3 to the developer roller 4 can be interrupted as shown in FIG. 5 with the screening member 6 rotated by approximately 90.degree. from the position shown in FIG. 4 when the developing device is not being used for development. A partitioning member 7 is placed between the stirrer roller 3 and the transporting magnet roller 5. Both the partitioning member 7 and the developer tank 1 are provided with a sealing member 8 made of an elastic material in contact with the screening member 6 for preventing the developing agent from leaking through the gaps between the screening member 6 and the partitioning member 7 and between the screening member 6 and the developer tank 1.
With a developing device thus structured, a strong driving force is required to rotate the screening member 6 because the sealing members 8 are in contact with it. There is also the possibility of the sealing members 8 becoming pulled into the gaps between the screening member 6 and the partitioning member 7 or between the screening member 6 and the developer tank 1, preventing the rotation of the screening member 6. Without the sealing members 8, on the other hand, stains may result on the photoreceptor since developing agent remains on the transportation magnet roller 5 even when developing is not taking place.